Organic foams based on polystyrene, polyolefin and polyurethane are very useful for thermal and acoustic insulation, but they are comparatively readily flammable and therefore unsuitable for many applications. BASOTECT, a foam based on melamine-formaldehyde resins and produced by BASF AG, has a high elasticity as a result of its open cells and its low density, which is of great advantage in handling and processing. Although elastic melamine-formaldehyde foams are not readily flammable, they are still combustible and thus do not meet the conditions of fire protection class A in accordance with DIN 4102. These conditions are naturally met by foams having an inorganic basis. However, the inorganic foams known hitherto all have a relatively high density and are thus not elastic. In addition, most of them are in-situ foams which are produced by, for example, injecting the components directly into hollow spaces on the building site and foaming and curing them there.
DE-A 21 65 912 describes a process for producing foams in which aqueous silicate solutions together with acid-releasing hardeners are foamed with the aid of volatile organic blowing agents and cured. The density of the open-celled foams varies from 20 to 900 g/l. The lowest density reported in the examples is 40 g/l. Elastic foams are not mentioned.
DE-A 32 44 523 describes the production of in-situ foams produced by mixing an alkali metal silicate solution with a hardener solution and a liquefied blowing gas under superatmospheric pressure. Hardeners used are acid-releasing carboxylic esters; the lowest density reported in the examples is 40 g/l. Both documents describe pure silicate foams which are not sufficiently moisture-resistant for practical use.
DE-A 36 17 129 likewise describes the production of in-situ foams. In the process described here, an aqueous silicate solution is foamed in the presence of a hardener by means of a gas which has been generated in a chemical reaction from a gas generation system, e.g. hydrogen peroxide. Hardeners used include, inter alia, alkali metal aluminate. The density of the in-situ foam varies from 30 to 1000 g/l.
According to WO 89/05285, a ceramic composition is produced by foaming an aqueous mixture of alkali metal silicate, alkali metal aluminate, a refractory ceramic material and aluminum powder, which generates hydrogen gas to act as blowing agent. The density of the ceramic composition is in the region of 500 g/l.
Finally, according to EP-A 407 921, a mixture of alkali metal water glass and a filler, e.g. aluminum oxide, is foamed by means of a chemical blowing agent, preferably azodicarbonamide, to give an open-celled, cured foam body having a density in the range from 50 to 500.